Continued growth of so-called "fast food"-style restaurants has increased interest in automating, to the greatest extent possible, the handling and cooking of food so that labor costs and customer waiting time can be reduced. One system for automating the cooking of pre-cut or discrete frozen food products, such as french fried potatoes, is described in U.S. patent application Ser. No. 07/759,606 filed Sep. 13, 1991, now U.S. Pat. No. 5,142,768 which is a continuation of U.S. patent application Ser. No. 07/458,982 filed Dec. 29, 1989 and now abandoned, both applications (hereinafter the "Prior Applications") having been assigned to the present assignee and are incorporated herein by reference. As a part of the automation process attention must be given to preserving the food product in a frozen state until just prior to cooking, when the product should be quickly thawed. On the one hand, modern commercial-scale frying operations work best and most efficiently when there is an ample and continuous supply of fully-thawed food product awaiting cooking. On the other hand, waste and the dangers of contamination and spoilage are reduced by not thawing a batch of frozen food product until immediately prior to cooking.
In the past, these inherently conflicting objectives were often compromised by simply transferring the food product directly from freezer to a "slacking rack", where the food product is disposed in a fry basket, and the latter placed on a rack. The food product is allowed to thaw under ambient conditions, usually near the frypots where the food is to be cooked. Once thawed the basket can be removed from the rack and placed in a frypot. This process relies on human intervention, and thus the appropriate thaw time must be watched; and once the food has thawed, if the operator is not distracted, the basket with the thawed product is removed and placed in a frypot. If the fry products are not completely thawed, at least portions of the product are too cold; while waiting too long to put the basket into the hot oil may result in at least portions of the product being too warm (and in some cases, the food product may become contaminated or spoil). Both circumstances obviously negatively impact the maximum throughput of the frying system since the frying process is dependent in part on the amount of product and the original temperature of the product. Thus, using a slacking or thawing rack to thaw frozen food product before frying it can cause uneven cooking and inferior taste and texture of the final cooked product.
It is therefore desirable to design an automatic thawing system, which is easily used with automated frying systems, such as the one described and claimed in the Prior Applications. The automated thawing system should provide the same degree of thaw as provided by standard slacking racks in at least the same amount of time, but consistently provide the entire thawed product at substantially the same temperature at which maximum throughput of the frying system is maintained. To be truly compatible with modern automated frying systems, an automated thawing system must be capable of thawing 1 to 2 lbs. of food product, such as french fried potatoes, every 20 to 30 seconds without adversely impacting the maximum throughput of common frypots, such as the model MJH-50 manufactured by Frymaster of Shreveport, La. The food product input to the thawing unit would typically be 1 to 2 lbs. of frozen food product at -10 to 0.degree. F. The food product must be rapidly thawed to conditions just above the frozen state, i.e. the thawed product must not be rigid and should have a surface temperature of about 40.degree. F.
In general, the prior art shows a variety of methods and apparatuses for warming, thawing or drying different types of food products. Several prior art patents show the general concept of using warmed air flows for accelerating the thawing or drying process. Several prior art patents also suggest the benefits of a multi-stage approach to the thawing or drying process.
For example, U.S. Pat. No. 4,228,730 (Schindler et al.) teaches an automated apparatus for continuous cooking of frozen french fried potatoes. In Schindler et al., frozen fries are held in a storage hopper (FIG. 3, reference numeral 120) from which they are fed by an upwardly slanting, endless conveyor into an enclosed bath of hot cooking liquid. Thus, a mechanical feed brings frozen fries directly to the cooking oil with little or no thawing. At col. 10, lines 50-53, Schindler et al. specifically state that "frozen pieces of potato are dropped into a hot cooking liquid. . .", thereby negating any inference that substantial pre-cook thawing might occur in this process.
U.S. Pat. No. 4,385,075 (Brooks) is directed to an apparatus for and method of thawing frozen foods. Brooks teaches the use of a special thawing compartment in combination with a conventional refrigerator appliance having both cooling and freezer sections. In Brooks an electric fan circulates air through the thawing compartment to facilitate thawing of food removed from the freezer section. The air is at a temperature of between 33.degree. F. and 55.degree. F., the ambient conditions of the cooling section of the refrigerator. The advantage of Brooks over similar prior art devices is that it avoids food spoilage after thawing and "uniformly thaws the food load without heating of the food load before complete thawing is achieved. . ." (col. 2, 11. 17-18). Brooks clearly does not envision use of an above ambient temperature air stream for thawing. This patent does not achieve "rapid" thawing and does not utilize a multi-stage approach.
U.S. Pat. No. 4,674,402 (Raufeisen) teaches a rather complex bakery apparatus designed to: (1) thaw frozen bakery goods using heated air; (2) cool the bakery goods to retard curing; and, (3) heat and humidify the bakery goods for proofing, all in a single system thereby eliminating the need to move the bakery goods between multiple processing units. The frozen bakery goods are placed on a stationary tray rack or tray cart (FIG. 4, reference numeral 29) positioned inside the apparatus. The thawing means of this invention comprises one or two thermostatically-controlled thawing heaters, "preferably of the electrical resistance-type" (col. 3, 1. 41).
U.S. Pat. No. 4,763,572 (Kuehl) is directed to a multi-stage apparatus for reducing the moisture content of liquid honey. The honey enters the apparatus at an upper inlet port and moves by gravity flow across a series of zig-zagged sloping trays while a fan circulates warm air to absorb moisture from the honey. There is no suggestion of using such an apparatus for thawing frozen foods.
U.S. Pat. No. 3,408,920 (Smith) is directed to a two-stage apparatus for rapid roasting of coffee beans. Speed of operation and "rapid turnover of the beans," (col. 1, 11. 65-69) are explicitly described as an objective of this invention. In the Smith apparatus, unroasted or green beans are loaded by gravity feed from a hopper (FIG. 1, reference numeral 14) into a first, upper cylindrical shell where they are fluidized in a counter-current flow of heated roasting gases. When roasting is completed, "the roasted beans. . . flow by gravity. . ." into a second, lower cylindrical shell for the cooling step (col. 7, 11. 3-14). In the lower shell, the roasted beans are sprayed from above with a coolinq fluid, while also being subjected to fluid flow from below. Thus, this patent teaches a sequential heating and cooling process, not a two-stage thawing process.
U.S. Pat. No. 4,750,416 (Graham) is directed to an apparatus for air-treating food products in conjunction with a spiral conveyor The objective of the air treatment may be "drying, heating, cooking, cooling and freezing. . ." (col. 1, 11. 16-17). U.S. Pat. No. 4,874,914 (Eke) is directed to a microwave oven that defrosts a frozen food item in multiple stages involving a forced air flow through the oven cavity. Finally, U.S. Pat. No. 4,803,961 (Hiraoka et al.) is directed to an air suction device for a multicylinder engine.
Thus, none of the apparatuses or methods described in the above cited patents is specifically adapted, or even suitable, for the type of rapid, efficient, and continuous or semi-continuous operation required to be effectively compatible with modern, automated, commercial-scale deep frying operations. These and other problems with and limitations of the prior art are overcome with the rapid frozen food thawing system of this invention.